Thousands of miles of buried natural gas pipes of varying size and formed from various materials are presently in service. All of these mains are in some state of progressive degradation. In most instances, the extent of such degradation is unknown, and hence, the serviceability of the mains is similarly unknown. This lack of information with the respect to the degree of degradation results in unforeseen gas pipe leaks and/or breaks, and necessitates the expending of substantial time and expense in locating these defects so that repairs and/or replacement can be made. Because of the need to detect conditions which might result in gas pipe breaks and/or leaks, an apparatus has been developed for inspecting gas pipes, and such apparatus is usually referred to as a pipe line "pig." Such pipe line "pigs" typically include a housing with a plurality of sensors, such as ultrasonic transducers, mounted to the outer surface thereof in a predetermined configuration or array to contact the inner surface of the gas pipe. As a "pig" moves axially down a gas pipe, the ultrasonic transducer associated therewith produces interrogation pulses which pass through a coupling medium and intercept the surfaces defining the inner diameter and the outer diameter of the pipe and/or any imperfections or flaws within the wall of the gas pipe. The surfaces defining the inner diameter and the outer diameter of the pipe and any imperfections or flaws within the wall of the pipe, in turn, cause the individual return pulses to be transmitted back to the ultrasonic transducer. By knowing the speed of sound in the different mediums through which the interrogation pulse travels (i.e., the coupling medium between the transducer and the pipe wall and the pipe wall itself), the thickness of the wall of the gas pipe can be computed by timing the difference between the return pulse from the surface defining the inner diameter of the pipe wall and the return pulse from the surface defining the outer diameter of the pipe wall. A more thorough discussion of these principles is found in U.S. patent application Ser. No. 08/222,621, filed on Apr. 5, 1994, and entitled "Scan Assembly Structure", the disclosure of which is incorporated herein by reference.
However, because the walls of the gas pipe contain imperfections and are often littered with debris, interrogating pulses transmitted into the wall of the gas pipe from a transducer reflect and refract off the debris and/or the imperfections of the wall at oblique or random angles, scattering the transmitted ultrasonic energy sufficiently such that little to no return pulses are received back at the transducer. The return pulses which are received are often times below the noise threshold and do not register as a return pulse. In addition, because of acoustical (i.e., mechanical) impedance mismatches at the interfaces of the transducer and coupler medium as well as at the interface of the coupler medium and the surface defining the inner diameter of the gas pipe, a portion of the transmitted ultrasonic energy is reflected at these interfaces rather than being transmitting across the interface, thereby even further reducing the amplitude of the return pulse. Accordingly, without an adequate ratio of energy received to energy transmitted, the effectiveness of the ultrasonic inspection is inhibited. This is of critical concern with "pigs" or scan assemblies which are robotic and have energy conservation issues such that merely increasing the amplitude of the interrogating pulse is not a viable option. Moreover, increasing the amplitude typically results in greater distortion of the interrogating pulse and increases reverberation noise, both of which decrease the signal-to-noise ratio and the accuracy of the inspection.
In view of the foregoing, it would be desirable to develop a method of inspecting the walls of gas pipes while under operating flow conditions whereby the energy efficiency of the interrogating pulse is maximized so as to improve signal-to-noise ratio and accuracy of the inspection.